The present invention relates to an improvement of an open-type fuel burner, and more particularly to such a fuel burner having a safety device for alerting the user or shutting off the supply of fuel when the oxygen content of air decreases below a prescribed level.
Fuel burners of the open-type find widespread use in applications where the portability and cost of a heating appliance are of primary concern. However, the recirculation of the air heated by the burner in a closed environment eventually depletes the oxygen content of the air in it and for this reason an oxygen sensor is provided to give a warning indication or automatically shut off the supply of fuel when the oxygen content falls below a predetermined level.
U.S. Pat. No. 4,315,729 issued to Tanaka et al, and assigned to the same assignee as the present invention, discloses a multistage gas burner having an inner tube and an outer tube arranged so that the inner tube mixes gas with primary air and produces a primary combustion flame at its outlet end and the outer tube mixes unburned gas with secondary air to cause a secondary flame to burn at its outlet end. A sensor is provided between the outlet ends of the inner and outer tubes to detect the absence of the primary combustion to trigger a safety device.
Because of the portability of the heating appliance of the above-mentioned type, it is desired that the size of the burner be kept as small as possible. Attempts at reducing the burner size, however, have resulted in an increase in temperature in the various parts of the burner, producing harmful products (NOx), "backfire" and unstable combustions.
In the aforesaid patent, the inner tube has a primary air inlet port located adjacent to the gas nozzle. However, the airstream drawn into the inner tube through the primary port by the ejected gas comes into a violent contact with the gas flow at the center of the inner tube and produces a mixture of air and gas which tends to tilt against the wall of the inner tube, thus resulting in a loss of uniformity in the distribution of gas and oxygen molecules in the mixture. As a result, the primary flame increases its length, causing the separation between it and the secondary flame ambiguous, which makes it difficult for the sensor to distinguish between normal and dangerous conditions. This problem can be cured only at the expense of size reduction.